1. Field of the Invention
The present invention relates to diamond drag bits.
More particularly, this invention relates to polycrystalline diamond compact (PDC) drag bits for drilling soft, sticky clay and shale earthen formations.
2. Description of the Prior Art
Earthen formations, such as bentonitic shales and other hydratable clays, that are plastic and sticky, are very difficult to drill because the drilled cuttings tend to coagulate and adhere to or "ball-up" the cutting face of the drill bit. This drastically reduces the drilling rate and bit life.
Roller cone drill bits and tungsten carbide "fish-tail" drag type drill bits have had limited success when attempting to drill these formations with water base muds. Both bit types "ball-up" very easily, severely slowing or stopping the drilling rate. This results in having to make numerous costly trips of the drill string to change bits.
Natural diamond drill bits also have had limited success drilling these sticky formations because they are very easily "balled-up" due to the extremely small protrusion of the diamond cutting elements.
PDC type drag bits in present use are very effective drilling soft, hydratable shales and clays when using oil base drilling mud, but severely "ball-up" when using water base drilling muds which hydrate the formations which made them sticky.
State of the art PDC drill bits for drilling soft formations are multiple bladed with PDC cutters affixed to the outer surfaces of the blades. The aforesaid blades have a leading side and a trailing side which are essentially vertical and parallel to the tilt axis. A single nozzle is positioned in relatively close proximity to the bit center and in the center of a fluid channel formed by two of the blades. The drilling fluid exiting the nozzle naturally flows radially at high velocity to the outer diameter of the bit close to the center line of the fluid channel. This creates low fluid pressure areas proximate the leading and trailing sides of adjacent blades, thereby inducing reverse flow of drilling fluid and entrained hydrated drill cuttings close to the blades. The hydrated drill cuttings now have an affinity for the metal bit head surface because of their electrical charge, therefore they aggregate and adhere to the bit head surface behind the trailing side of the aforesaid preceding blade. The rotation of the bit while drilling also causes a differential pressure between the leading and trailing sides of the blades amplifying the adherence of the drill cuttings to and subsequent balling of the bit head.
A new fluid dynamics control mechanism is disclosed which overcomes the inadequacies of the prior art. This new control can be adapted to the basic blade type bits presently in use.